Sometimes software glitches occur if you go back to do something BEFORE finalizing the transaction, such as looking at other payment methods or updating a payment method (such as changing expiration date or security code).
I've had my shipping address and billing address switched by going back to check something before clicking the Purchase button, the total purchase amount being different after leaving the finalize transaction page, etc.
If I ever have to make a change to anything after somethingis in my cart, I'll redo the purchase, including closing the browser first.
